As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for batteries has also sharply increased as an energy source for the mobile devices. Accordingly, much research on batteries satisfying various needs has been carried out.
In terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries, which are thin enough to be applied to products, such as mobile phones, is very high. In terms of the material for batteries, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, having high energy density, high discharge voltage, and high output stability, is very high.
FIG. 1 is an exploded perspective view typically illustrating the general structure of a conventional representative pouch-shaped secondary battery.
Referring to FIG. 1, the pouch-shaped secondary battery 10 includes an electrode assembly 30, pluralities of electrode taps 40 and 50 extending from the electrode assembly 30, electrode leads 60 and 70 welded to the electrode taps 40 and 50, respectively, and a battery case 20 for receiving the electrode assembly 30.
The electrode assembly 30 is a power generating element comprising cathodes and anodes successively stacked while separators are disposed respectively between the cathodes and the anodes. Specifically, the electrode assembly 30 is constructed in a structure in which the electrode leads 60 and 70 are electrically connected to the electrode taps 40 and 50 extending from the cathodes and the anodes, respectively, for example, by welding. The electrode assembly 30 is mounted in the battery case 20 while the electrode leads 60 and 70 are partially exposed to the outside of the battery case 20. To the upper and lower surfaces of the electrode leads 60 and 70 are partially attached insulative films 80 for improving the sealability between the battery case 20 and the electrode leads 60 and 70 and, at the same time, for securing the electrical insulation between the battery case 20 and the electrode leads 60 and 70.
FIG. 2 is an enlarged view, in section, illustrating the upper end of a battery case of the secondary battery shown in FIG. 1, in which cathode taps are coupled to each other in a concentrated state and connected to a cathode lead, after the assembly of the secondary battery. For convenience of description, only the cathode lead is shown in FIG. 2. The structure of the cathode lead is identical to that of the anode lead.
Referring to FIG. 2, the plurality of cathode taps 40, which extend from cathode current collectors (not shown) of the electrode assembly 30, are coupled to each other, for example, by welding, and connected to one end of the cathode lead 60. The cathode lead 60 is thermally welded to an upper sealed portion 21 of the battery case 20, together with the insulative films 80, while the other end 61 of the cathode lead 60, which is opposite to the cathode taps 40, is exposed to the outside of the battery case 20. Two insulative films 80, attached to the cathode lead 60, overlap with each other at opposite major surfaces of the cathode lead 60, while the insulative films 80 have the same length. Consequently, upper ends 81 and the lower ends 82 of the insulative films 80 structurally coincide with each other. Generally, PP or PE having a thickness of approximately 80 to 100 □ is used as the insulative films. When the thermal welding process is performed, the insulative films are integrally fixed to the battery case 20 at the upper sealed portion 21 of the battery case 20.
When the electrode assembly 30 (see FIG. 1) frequently moves in the battery case due to various reasons, such as the application of small vibrations to the battery or the dropping of the battery, a region A of the cathode lead 60 corresponding to the front ends of the cathode taps 40, at the lower end of the cathode lead 60, to which the plurality of cathode taps 40 are attached, may break. Specifically, the region of the cathode lead 60 where the insulative films 80 are attached is protected from external force, whereas stress is concentrated on the region A of the cathode lead 60, which is frequently deformed due to the external force.
In addition, the insulative films 80 protrude partially outward from the upper sealed portion 21 of the battery case 20, and the insulative films 80 are attached to the opposite major surfaces of the cathode lead 60 at the protrusion region B in a symmetrical fashion. As a result, when the upper sealed portion 21 of the battery case 20 is bent to reduce the total length of the secondary battery 10 (see FIG. 1) or mount an element, such as a protection circuit module (not shown), it is difficult to bend the region B of the cathode lead 60 due to the insulative films 80, attached to the opposite major surfaces of the cathode lead 60.
In this connection, Japanese Patent Application Publication No. 2003-257387 discloses a secondary battery constructed in a structure in which an electrode assembly having a cathode/separator/anode arrangement is mounted in a battery case, pluralities of electrode taps, protruding from the electrode assembly, are connected to electrode leads, at least one of insulative films attached to opposite major surfaces of each electrode lead extends to a bent region where the electrode taps are bent to prevent a short-circuit of the electrode taps to the electrodes of the electrode assembly. Also, Japanese Patent Application Publication No. 1999-260414 discloses a secondary battery constructed in a structure in which thermally weldable polymer films are formed at the surfaces of electrode current collectors protruding outward from a battery case, and the polymer films are thermally welded to the battery case, the inner surface of which is made of thermally weldable polymer film, thereby improving the sealability of the battery case.
However, the above-described technologies do not suggest a structure to prevent the breakage of the electrode leads at the lower interfaces of the insulative films or solve the difficulty in bending the electrode leads at the upper protrusion regions of the insulative films.
Consequently, there is a high necessity for a technology that is capable of improving the structure of the insulative films thermally welded to the upper sealed portion of the battery case, thereby preventing the breakage of the electrode leads due to the movement of the electrode assembly when external forces, such as small vibrations and dropping, are applied to the battery, and, preferably, easily bending the electrode leads such that the PCM is mounted to the battery.